Nobody can tell you why Apple has not included some feature you want, but you can ask for it via http://www.apple.com/feedback In the meantime you can add dictionaries available for OpenOffice if yoiu find one for the language you want,

I'd love to know when they're going to add Hindi as an OS X interface language -- and more English variants, especially when Windows, Linux distributions and Apple's other products include UK English at the very least. (I know you're not Apple, I'm just dumbfounded at the absence of these specific language groups, especially when you've got iOS which has just added Hindi, and has about four flavours of English.)